Laser systems including fiber amplifiers are commonly used in many applications, including telecommunications applications and high-power military and industrial fiber optic applications. In operation, the propagating optical signal from a laser source is introduced in the core region of a section of optical fiber and is amplified through the use of an optical “pump” signal. The pump is of a predetermined wavelength that will interact with particular dopants included in the core region of the fiber amplifier (typically rare earth materials, such as erbium, ytterbium, or the like) to amplify the propagating optical signal.
High power fiber amplifiers are often limited in power, however, as a result of the unwanted creation of stimulated Brillouin scattering (SBS). That is, the strong optical signal is scattered in the backward direction by thermally-generated acoustic waves (i.e., thermal Brillouin scattering) in the core of the optical fiber. This backscattered light is down-shifted in optical frequency (Stokes scattering) from the incident light by the Brillouin shift frequency ΩB (rad/sec) which is determined by the optical wavelength, the core refractive index and the sound speed in the core. The Stokes-shifted, backward-propagating light combines with the original forward-propagating signal light to create a traveling periodic intensity pattern in the fiber core. This intensity pattern causes a traveling periodic modulation of the fiber density due to the electrostrictive effect, which is the tendency of a material to compress in the presence of strong optical intensity and, therefore, generates a forward-propagating, electrostrictively-generated sound wave similar to (and with the same sound speed as) the acoustic wave that caused the original light scattering event. This modulation reinforces the scattering process seeded by the original thermal Brillouin scattering event, thereby generating “stimulated Brillouin scattering”, or SBS, in optical fibers. The reinforcement occurs via two different mechanisms: (1) the electrostrictively-generated sound wave creates additional scattering at the same wavevector and frequency, and (2) the electrostrictively-generated pressure will mechanically drive the acoustic phonon that generated the original thermal Brillouin scattering. The SBS energy travels in a backward direction and is shifted in frequency proportional to both the acoustic velocity and refractive index of the fiber. In one typical arrangement, the signal light is downshifted in frequency by about 15 GHz at an optical wavelength of 1083 nm.
The threshold condition for SBS can be written as:
                                          P            th                    =                                                    21                ⁢                                  A                  eff                                                                              g                  B                                ⁢                L                                      ⁢                          (                              1                +                                  BW                                      BW                                          SiO                      2                                                                                  )                                      ,                            (        1        )            where Aeff is the effective mode area of the fiber, gB is the Brillouin gain coefficient, L is the length of the fiber, BW is the bandwidth of the signal and BWSiO2 is the Brillouin bandwidth of a silica fiber.
In extreme cases, the back-reflected SBS energy robs power from the signal and clamps the output power. For high power rare earth fiber amplifiers, the back-reflected light is then further amplified by the rare earth material in the core region and can result in very high intensity backward propagating pulses that destroy the fiber or other upstream optical components. Increasing the performance of fiber amplifiers thus requires the reduction of SBS.
One technique for reducing the onset of SBS is to increase the area of the optical mode. As shown above, the SBS power threshold Pth scales with effective mode area Aeff, since the optical intensity is reduced as the area increases. As a result, many manufacturers of specialty fiber for high power amplifiers produce fibers with large core diameters (on the order of, for example, 15-30 μm). However, increasing the core diameter beyond about 25 μm will increase bend loss and mode coupling, degrading the quality of the propagating optical signal.
Another approach to increasing the SBS threshold is to alter the acoustic field distribution in the fiber core. In a typical small mode area (SMA) fiber, the acoustic velocity of the core material is less than that of the surrounding cladding layer and therefore the acoustic refractive index is higher in the core region, causing the acoustic mode to be guided by the core, just as the optical field is guided. The resulting high spatial overlap between the optical and acoustical fields enhances the unwanted interaction and results in strong seeding of the SBS process via thermal Brillouin scattering. Moreover, the electrostrictively-driven acoustic field generated in the central core region occupied by the optical mode is guided by the acoustic waveguide thereby further enhancing SBS generation.
It has been found that by altering the composition of the core and cladding so that the acoustic velocity of the core is greater than that of the cladding, the acoustic mode may be excluded from the central core region occupied by the optical mode. This exclusion of the acoustic mode will thereby reduce the thermal Brillouin scattering that seeds the SBS process. In addition, any generated acoustic field in the central core region will sample the core-cladding interface and refract out of the anti-guiding structure. FIG. 1 is a prior art illustration of this particular arrangement for a conventional small mode area (SMA) fiber, which shows the refractive index profile of such a fiber 10 with a composition selected so that the acoustic velocity of the core 12 is greater than that of the cladding 14. Both the optical and acoustical refractive index profiles are shown in FIG. 1. The acoustic index profile excludes thermal phonons from the optical core region and the anti-guiding structure causes acoustic energy to radiate out of the core region, as shown by arrows “A” in FIG. 1. As a result, the SBS threshold will be improved by more than a factor of two.
FIG. 2 illustrates in particular the diffraction of an SBS-generated acoustic wave in the SMA fiber of FIG. 1. As mentioned above, the optically-induced acoustic wave is generated by the electrostrictive effect and is represented by a plane wave P and an aperture A. The presence of the circular aperture whose diameter D is chosen to be approximately equal to the 1/e points of the acoustic intensity distribution (for a Gaussian optical mode) causes the acoustic wave to diffract as it propagates beyond the aperture. The nature of the diffraction at a distance L from the aperture is governed by the Fresnel number, and the acoustic sound wave will exhibit a finite lifetime due to the conversion of the acoustic energy into heat within the fiber's glass material. The Fresnel number evaluated at the known phonon decay length Lph (38 μm) will have a value of 0.32 (thus, is less than one). Having a value of less than one, the acoustic wave undergoes far-field (Fraunhofer) diffraction in the manner shown in FIG. 2. As shown, the acoustic intensity distribution begins near the aperture to cover a region essentially the same as the core diameter. As the acoustic wave propagates during its lifetime, it spreads out (diffracts) and samples the core-cladding interface, as well as regions of the inner cladding beyond the core-cladding interface. Therefore, an acoustic index structure designed to suppress the onset of SBS in SMA fibers must be located in the region of the fiber sampled by the acoustic wave, in this case indicated by the shaded box in FIG. 2.
However, in large mode area (LMA) fibers, the optical field lies well within the core region, as shown in FIG. 3. An LMA fiber 11 is shown as comprising a relatively large diameter core region 13 and surrounding depressed cladding area 15. As shown, a diffracting acoustic ray (indicated by arrows “B”) will remain within core area 13 and be unable to sample the core-cladding interface 17 of LMA fiber 11 since the distance to the interface exceeds the decay length Lph of the phonon.
FIG. 4 shows the acoustic diffraction of an SBS-generated sound wave within the LMA fiber 11 of FIG. 3. In general, an LMA fiber will have a core diameter (CD) greater than its mode field diameter (MFD), where it is presumed that CD=1.4*MFD. As with the illustration of FIG. 2, the SBS acoustic wave in the LMA fiber of FIG. 4 is represented by a plane wave P-L and an aperture A-L with an aperture diameter of D. For this arrangement, the calculated Fresnel number is 3.6—greater than one—and in that case corresponds to near-field (Fresnel) diffraction. Therefore, the acoustic energy lies within a radius defined by the aperture radius for the lifetime of the sound wave. Referring to FIG. 4, it is shown that the sound wave continues to propagate within the core without any appreciable spreading into the core-cladding interface—in contrast to the spreading associated with the SMA fiber shown in FIG. 2.
In summary, therefore, the various arrangements of the prior art cannot be configured to simultaneously provide large optical mode field and an effective anti-guiding acoustic structure. A need thus remains in the art for an arrangement that provides the reduction of the presence of SBS in LMA fibers, without compromising the high power performance of the LMA fiber itself.